Thermal block of mammalian unmyelinated C fibers by local cooling to 15-25°C after a brief heating at 45°C

Abstract

The purpose of this study was to examine the changes in cold block of unmyelinated C fibers in the tibial nerve by preconditioning with heating and to develop a safe method for thermal block of C-fiber conduction. In seven cats under α-chloralose anesthesia, C-fiber-evoked potentials elicited by electrical stimulation were recorded on the tibial nerve during block of axonal conduction induced by exposing a small segment (9 mm) of the nerve to cooling (from 35°C to ≤5°C) or heating (45°C). Before heating, partial, reproducible, and reversible cold block was first detected at a threshold cold block temperature of 15°C and complete cold block occurred at a temperature of ≤5°C. After the nerve was heated at 45°C for 5-35 min, the threshold cold block temperature significantly (P ≺ 0.05) increased from 15°C to 25°C and the complete cold block temperature significantly (P ≺ 0.05) increased from ≼5°C to 15°C on average. The increased cold block temperatures persisted for the duration of the experiments (30 -100 min) while the amplitude of the C-fiber-evoked potential measured at 35°C recovered significantly (P ≺ 0.05) to ~80% of control. This study discovered a novel thermal method to block mammalian C fibers at an elevated temperature (15-25°C), providing the opportunity to develop a thermal nerve block technology to suppress chronic pain of peripheral origin. The interaction between heating and cooling effects on C-fiber conduction indicates a possible interaction between different temperature-sensitive channels known to be present in the mammalian C fibers.